The present invention relates to a compact, low-insertion loss surface acoustic wave (SAW) filter having a desired frequency characteristics and a portable telephone utilizing the same.
There are a number of SAW filters having acceptable frequency characteristics proposed in the past. For example, in Japanese Patent Application Laid-Open (Kokai) No. 154917/1983, discloses a method of realizing a low-insertion loss filter in connection with a number of interdigital transducers (IDTs) arranged in parallel to a propagating direction of surface acoustic wave, i.e. a direction perpendicular to electrode fingers thereof, and connected alternately to an input terminal and an output terminal of the filter, in which the number of electrode fingers forming the interdigital transducers in a center portion thereof is greater than those of electrode fingers of the opposite side portions thereof so that the number of interdigital transducers is reduced.
An article "SAW Filters Employing Interdigitated Interdigital Transducers, IIDT", M. Lewis, 1982 ULTRASONIC SYMPOSIUM, pages 12 to 17, IEEE discloses similar filters which realize easy fabrication and low insertion loss. Another article, "LOW LOSS SAW FILTER FOR ANTENNA DUPLEXER", M. Hikita, et al, 1983, ULTRASONIC SYMPOSIUM, pages 77 to 86, IEEE discloses a filter similar thereto which is effectively applied to a portable telephone.
Although the filters disclosed in these articles are superior in simplicity of the related structure and in low insertion loss compared with conventional filters in which two or three IDTs are used, reduction of loss and flatness of frequency characteristics are not always enough.
An object of the present invention is to provide a compact, low insertion loss surface acoustic wave filter having sufficiently flat frequency characteristics to thereby solve the problems of the prior art.
Another object of the present invention is to provide a portable telephone which utilizes the compact, low insertion loss surface acoustic wave filter as a duplexer or as a bandpass filter in a first stage of the telephone.